Optical system for recording and/or reproducing an optical information recording medium

Abstract

An optical system for recording or reproducing information in an optical information recording medium, includes: a light source having a wavelength λ thereof; an objective lens; tracking device for tracking the objective lens by moving the objective lens in a direction perpendicular to an optical axis of the optical system; and a coupling lens optical system including at least one plastic lens and having a positive focal length, and being provided between the light source and the objective lens for guiding a light emitted from the light source to the objective lens. A maximum amount of change in a wavefront aberration of the coupling lens optical system in a numerical aperture corresponding to a maximum amount of tracking is 0.02 λrms or less.

What is claimed is: 
     
       1. An optical system for recording or reproducing information in an optical information recording medium, comprising: (a) a light source having a wavelength λ thereof;   (b) an objective lens;   (c) tracking means for tracking the objective lens by moving the objective lens in a direction perpendicular to an optical axis of the optical system; and   (d) a coupling lens optical system including at least one plastic lens and having a positive focal length, and being provided between the light source and the objective lens for guiding a light emitted from the light source to the objective lens;   wherein a maximum mount of change in a wavefront aberration of the coupling lens optical system in a numerical aperture corresponding to a maximum amount of tracking is 0.02 λrms or less.   
     
     
       2. The optical system of claim 1, wherein a saturation water absorbing rate of the plastic lens included in the coupling lens optical system is 0.5% or less.   
     
     
       3. The optical system of claim 2, wherein the plastic lens included in the coupling lens optical system is made of either polyolefine resin or norbornene resin.   
     
     
       4. The optical system of claim 1, wherein the maximum amount of tracking of the tracking means is 0.1 mm to 0.7 mm. 
     
     
       5. The optical system of claim 1, wherein a numerical aperture NA 0  of the objective lens on a side of the optical information recording medium satisfies the following expression,   NA.sub.0 >0.52.     
     
     
       6. The optical system of claim 1, wherein the wavelength λ of the light source is 700 nm or less. 
     
     
       7. A coupling lens optical system for use in an optical system for recording or reproducing information in an optical information recording medium, the coupling lens optical system having a positive focal length and being provided between a light source having a wavelength λ and an objective lens which moves for tracking in a direction perpendicular to an optical axis of the optical system for recording or reproducing, for guiding a light emitted from the light source to the objective lens, the coupling lens optical system comprising: a plastic lens,   wherein a maximum amount of change in a wavefront aberration of the coupling lens optical system in a numerical aperture corresponding to a maximum amount of tracking is 0.02 λrms or less.   
     
     
       8. The coupling lens optical system of claim 7, wherein a saturation water absorbing rate of the plastic lens included in the coupling lens optical system is 0.5% or less.   
     
     
       9. The coupling lens optical system of claim 8, wherein the plastic lens included in the coupling lens optical system is made of either polyolefine resin or norbornene resin.   
     
     
       10. The coupling lens optical system of claim 7, wherein a numerical aperture NA 0  of the objective lens on a side of the optical information recording medium satisfies the following expression,   NA.sub.0 >0.52.       
     
     
       11. The coupling lens optical system of claim 7, wherein the wavelength λ of the light source is 700 nm or less.   
     
     
       12. The coupling lens optical system of claim 7, wherein the coupling lens optical system is a plastic single lens.   
     
     
       13. The coupling lens optical system of claim 12, wherein a saturation water absorbing rate of the plastic single lens is 0.5% or less.   
     
     
       14. The coupling lens optical system of claim 12, wherein the plastic single lens is made of either polyolefine resin or norbornene resin.   
     
     
       15. The coupling single lens of claim 12, wherein a focal length f c  of the plastic single lens satisfies the following expression,   12 mm<f.sub.c <36 mm.